1. Field of the Invention
The present invention relates to a digital xerographic color image forming apparatus for forming a color image by repeating charging of a photosensitive medium, exposure of the photosensitive medium to a light beam according to digital data for formation of an electrostatic latent image, and development of the electrostatic latent image plural times to thereby form toner images of a given number of colors on the photosensitive medium, and then collectively transferring these toner images of the given number of colors onto a recording medium.
2. Description of the Related Art
Such a digital xerographic color image forming apparatus is disclosed in Japanese Patent Laid-open No. 59-121348, for example. The technical background section of Japanese Patent Laid-open No. 59-121348 includes a description regarding a fundamental color electrophotographic system for forming a color image by repeating charging, exposure, and development to a photosensitive member by times corresponding to a required number of colors to thereby form toner images of the required number of colors in a layered or registered condition on the photosensitive member, and then collectively transferring all the toner images onto a recording medium. Of such a color electrophotographic system, there is a specific system including a required number of developing units corresponding to the required number of colors, a single charging unit, and a single exposing unit, wherein the single charging unit and the single exposing unit are repeatedly used by rotation of the photosensitive member. Accordingly, this specific system has a decreased number of components, thereby allowing a reduction in machine size and a reduction in cost. In addition, since the toner images of plural colors are formed in a registered condition on the single photosensitive member, misregistration of picture elements of the plural colors can be reduced with the result that a high-quality color image with less misregistration in color can be provided in principle. In the color electrophotographic system mentioned above and similar color electrophotographic systems in the related art to be mentioned hereinafter, the exposure for the second and next colors is applied on the same area of the photosensitive member as that for the first color, and the toner images of the second and next colors are registered with the toner image of the first color, thus forming all the toner images in the registered condition on the photosensitive member.
However, a color image forming apparatus employing such a system of forming a color image by registering the plural colored toner images on the photosensitive member has the following problem as known from a literature of "Color Electrophotographic Process; Registration of Toner Images on Photosensitive Member" (NIP-7, Japan Hardcopy, '89 collection of papers). That is, a so-called toner shielding effect such that a toner image previously formed on the photosensitive member hinders the exposure to be next performed and a rise in potential of charged toner layers due to sequential registration of the charged toner layers on the photosensitive member have an influence on a potential of electrostatic latent images to be formed on the photosensitive member, reducing tone reproduction and resolution. Further, such a color image forming apparatus also has the following problem as pointed out in a literature of "Color Electrophotographic Process by Color Registration Development" (Journal of the Society for Electrophotography, Vol. 30, No. 1, 1991). That is, a reduction in resolution is caused by a rise in potential of exposed portions such that a potential of the photosensitive member is not enough decreased even by the exposure because of electrostatic charges of the toner layers sequentially superimposed on the photosensitive member and by a dielectric property of the toner layers sequentially superimposed on the photosensitive member.
As described above, the color electrophotographic system for forming a color image by repeating charging, exposure, and development to the photosensitive member by times corresponding to a required number of colors to thereby form the toner images of the required number of colors in a registered condition on the photosensitive member, and then collectively transferring all the toner images onto the recording medium has excellent features that it can reduce a machine size and a cost and can provide a high-quality color image with less color misregistration in principle. On the other hand, however, the color electrophotographic system mentioned above also has some problems that the rise in potential of the toner layers, the dielectric property of the toner layers, and the toner shielding effect cause a deterioration in image quality, that is, a serious adverse effect on the image quality of the color image.
Of these problems adversely affecting the image quality of the color image, the problem caused by the rise in potential of the toner layers (or the rise in potential of the unexposed portions due to the charges of the toner layers) is most connected with the deterioration in image quality in the system of forming the color image by registering the plural colored toner images on the photosensitive member. It is therefore greatly important to solve this problem caused by the rise in potential of the toner layers or the like.
Some technical means of solving the problem caused by the rise in potential of the toner layers or the like have already been proposed in Japanese Patent Laid-open Nos. 63-88573 and 3-202869, for example. More specifically, Japanese Patent Laid-open No. 63-88573 discloses a color electrophotographic apparatus for forming plural toner images of different colors on a photosensitive member by repeating a cycle of charging, exposure, and development plural times, wherein there is provided means for stepwise increasing a charging potential of the photosensitive member with the repetition of the cycle. In more detail, a charging potential of the photosensitive member, a potential of an image area, and a voltage applied to a developing roller (i.e., a developing bias) are increased in the order of the first color (yellow), the second color (magenta), and the third color (cyan) as shown in FIG. 23, thereby making a contrast potential between an image area and a nonimage area in a toner clinging portion of the photosensitive member larger than that in a conventional system. Thus, the problem caused by the rise in potential of the toner layers or the like is intended to be solved.
On the other hand, Japanese Patent Laid-open No. 3-202869 mentioned above discloses a color image forming apparatus comprising an electrostatic latent image forming member, first charging means provided around the electrostatic latent image forming member, second charging means provided around the electrostatic latent image forming member, exposing means provided around the electrostatic latent image forming member, developing means provided around the electrostatic latent image forming member and storing different colored toner particles adapted to be charged with the same polarity as that of the electrostatic latent image forming member, control means provided around the electrostatic latent image forming member for controlling the polarity and a charge amount of the different colored toner particles on the electrostatic latent image forming member, and transferring means provided around the electrostatic latent image forming member, wherein the first charging means, the second charging means, the exposing means, the developing means, the control means, and the transferring means are arranged in this order along a direction of movement of the electrostatic latent image forming member, and the different colored toner particles on the electrostatic latent image forming member are reversely developed to be superimposed with each other. In this color image forming apparatus, a surface potential of the electrostatic latent image forming member having passed the first charging means is made larger than a surface potential of the electrostatic latent image forming member having passed the second charging means. In this manner, after the electrostatic latent image forming member is charged by the first charging means to acquire a first charging potential or more required for development, the charges on the electrostatic latent image forming member are erased by the second charging means so that a charging potential of the electrostatic latent image forming member becomes lower than the first charging potential, and the polarity of the toner on the electrostatic latent image forming member is reversed, thereby intending to eliminate a reduction in contrast potential at a color registered portion on the electrostatic latent image forming member and prevent toner splash upon exposure.
Further, various technical means of solving the problem caused by the toner shielding effect have already been proposed in Japanese Patent Laid-open Nos. 63-65460, 63-66579, 63-65459, 63-298360, and 3-290676, for example. According to these technical means, exposure conditions in forming images of the second and next colors are changed from previous exposure conditions.
Japanese Patent Laid-open No. 63-65460 mentioned above discloses a multicolor image forming method comprising the steps of uniformly charging an image forming member composed of a conductive substrate and a photoconductive layer formed on the conductive substrate, exposing the image forming member to light according to an image signal for a given color to form a latent image, reversely developing the latent image with a colored toner corresponding to the given color, repeating the steps of charging, exposing, and developing for plural predetermined colors in given order to thereby form a multicolor toner image on the image forming member, and transferring the multicolor toner image onto a transfer member to thereby form a multicolor image on the transfer member, wherein an exposure intensity in forming the latent image on the image forming member is sequentially increased as the steps of charging, exposing, and developing for the plural colors proceed.
Japanese Patent Laid-open No. 63-66579 discloses a multicolor image forming method comprising the same steps as those of the method disclosed in Japanese Patent Laid-open No. 63-65460, wherein an exposure time per picture element in forming the latent image on the image forming member is sequentially increased as the steps of charging, exposing, and developing for the plural colors proceed.
Japanese Patent Laid-open No. 63-65459 discloses a multicolor image forming method comprising the same steps as those of the method disclosed in Japanese Patent Laid-open No. 63-65460, wherein a light beam diameter for exposure in forming the latent image on the image forming member is sequentially increased by beam diameter control means as the steps of charging, exposing, and developing for the plural colors proceed.
Japanese Patent Laid-open No. 63-298360 discloses a color electrophotographic apparatus comprising image signal generating means for generating an optical image signal, a photosensitive member having a form of drum and adapted to rotate in a circumferential direction of the drum, the optical image signal generated from the image signal generating means being written on the photosensitive member to form electrostatic latent images, an image optical system for imaging the optical image signal on the photosensitive member, charging means for preliminarily uniformly charging the photosensitive member, a plurality of developing means for developing the electrostatic latent images formed on the photosensitive member by using different developers to thereby form toner images on the photosensitive member, transferring means for transferring the toner images onto paper, feed control means for feeding the paper in synchronism with rotation of the photosensitive member, photosensitive member cleaning means for removing a part of the developers left on the photosensitive member and not transferred onto the paper to clean the photosensitive member, fixing means for fixing the toner images transferred onto the paper, photosensitive member driving means for rotating the photosensitive member, and control means for controlling drive of the image signal generating means, selective operation for selecting one of the plural developing means, operation of the transferring means, control operation of the feed control means, operation of the cleaning means, operation of the fixing means, and operation of the photosensitive member driving means, wherein the control means determines a portion of the photosensitive member where the optical image signal is written and a portion of the photosensitive member where the optical image signal is not written when the image signal generating means generates the optical image signal for development by one of the plural developing means previously selected, stores write address information of the optical image signal into a memory, reads the write address information stored in the memory when the image signal generating means generates the optical image signal for development by one of the plural developing means next selected, determines a portion of the photosensitive member where the optical image signal is to be written and a portion of the photosensitive member where the optical image signal is not to be written, and controls a light quantity of the optical image signal.
Finally, Japanese Patent Laid-open No. 3-290676 discloses a multicolor recording apparatus for recording an image represented by at least first color information, first tone information, second color information, and second tone information in accordance with an electrophotographic system, wherein first exposure is performed according to the first color information and the first tone information to form a first electrostatic latent image, and second exposure is performed according to the second color information and the second tone information to form a second electrostatic latent image superimposed with the first electrostatic latent image with a light intensity for the second exposure being controlled according to the first tone information and the second tone information.
However, the techniques in the related art mentioned above have the following problems. In the color electrophotographic apparatus disclosed in Japanese Patent Laid-open No. 63-88573, the contrast potential between the image area and the nonimage area (i.e., the contrast potential of the latent image) for the second and next colors can be made larger than that in the conventional apparatus as shown in FIG. 23 (in connection with this, the contrast potential for the second and next colors in the preferred embodiments of the present invention is 750 V which is the same as the contrast potential for the first color as will be hereinafter described). However, a developer image contrast having a more influence on an actual image density is not constant for each color. That is, the developed image contrast for the first color is 650 V, but the developed image contrast for the second and next colors is 700 V as shown in FIG. 23. As a result, the developed image density for each color varies to render difficult color reproduction with a stable synthetic color. Further, although not described in Japanese Patent Laid-open No. 63-88573, the potential of toner layers to be formed on the photosensitive member actually largely changes with the weight of developing toners per unit area. According to a test made by the present inventors, the potential of toner layers to be formed on the photosensitive member actually changes with the weight of developing toners per unit area as shown in FIG. 24. As understood from FIG. 24, the potential of toner layers in the range of 0.5 to 1.0 mg/cm.sup.2 of the weight of developing toners which value is required for a high-quality color image becomes 60 to 120 V (absolute value), and the potential of the image area upon development for the third color rises up to a maximum value of 250 V or more corresponding to a value twice the potential of each toner layer, 120 V. Accordingly, in order to ensure a necessary latent image contrast (e.g., 750 V) in relation to the potential of the image area of the toner layers previously developed on the photosensitive member, the surface of the photosensitive member must be subjected to primary charging up to about 1000 V (i.e., the sum of the image area potential, 250 V and the latent image contrast, 750 V) upon image formation for the third color.
However, such primary charging of the surface of the photosensitive member to a high voltage of about 1000 V causes a partial deterioration of the photosensitive member to result in occurrence of whitening or black spots on the image. Additionally, such a technical means for sequentially increasing the charging potential of the photosensitive member with proceeding of the development cycles originally has no effect against the problem caused by the toner shielding effect. Accordingly, the exposure for the second and next colors is blocked by the toner layer previously formed on the photosensitive member, so that a desired contrast itself cannot be obtained. In particular, since a highlight area important for a high-quality color image has a low toner density and accordingly has a small weight of toner layers, the potential of toner layers on the photosensitive member in the highlight area is low as shown in FIG. 24. Accordingly, in the highlight area where the toner layers having a small weight are superimposed on the photosensitive member, the potential of the toner layer for the first color previously formed on the photosensitive member is low and a proportion of influence of this low potential to the potential of the image area for the second and next colors becomes relatively large. As a result, it is very difficult to faithfully reproduce a given tone at a given density in the highlight area.
In the color image forming apparatus disclosed in Japanese Patent Laid-open No. 3-202869, after the electrostatic latent image forming member is charged by the first charging means to acquire a first charging potential or more required for development, the charges on the electrostatic latent image forming member are erased by the second charging means so that a charging potential of the electrostatic latent image forming member becomes lower than the first charging potential, and the polarity of the toner on the electrostatic latent image forming member is reversed, thereby eliminating a reduction in contrast potential at a color registered portion on the electrostatic latent image forming member and preventing toner splash upon exposure. In this related art, the polarity of the toner on the surface of the electrostatic latent image forming member is reversed to cause a problem such that this toner reversed in its polarity jumps back to the developing unit for the next color to generate mixing of colors (so-called scavenging phenomenon). This phenomenon is remarkable especially when applying an AC bias component as a developing bias to the developing units for the second and next colors. In consideration of the fact that the application of the AC bias component to the developing units for the second and next colors is essential for the color image forming apparatus, this problem is fatal. In addition, the apparatus disclosed in Japanese Patent Laid-open No. 3-202869 has another problem such that the first and second, two charging means and two high-voltage power supplies are required to unavoidably cause an increase in size of the charging unit and an increase in cost.
On the other hand, in the technical means of solving the problem caused by the toner shielding effect as disclosed in Japanese Patent Laid-open Nos. 63-65460, 63-66579, 63-65459, 63-298360, and 3-290676, the exposure intensity, the exposure time per picture element, or the light beam diameter for exposure is sequentially increased as the steps of charging, exposing, and developing for the plural colors proceed; the light quantity of the optical image signal is controlled by determining a portion of the photosensitive member where the optical image signal is to be written and a portion or the photosensitive member where the optical image signal is not to be written; or the light intensity upon exposure for the second color is controlled according to the first tone information and the second tone information. All of these techniques can act as countermeasures against the problem caused by the toner shielding effect, but cannot act as countermeasures against the problem caused by the rise in potential of the exposed portions due to toner charges and the dielectric property of the toner layers. Thus, such techniques are not satisfactory in obtaining a high-quality color image.